1. Field of the Invention
The present invention relates to a valve spring steel having a strength as high as 210 to 240 kgf/mm.sup.2 after oil tempering.
2. Description of the Related Art
Although JIS generally specifies the basic chemical composition system of valve spring steels, a steel having a tensile strength as high as 210 to 240 kgf/mm.sup.2 cannot be ensured only by imitating the chemical composition system, and the steel thus obtained naturally has a limitation on the setting resistance. In contrast to the composition system, Kokai (Japanese Unexamined Patent Publication) No. 7-292435 discloses that composite addition of Si and Cr to a steel having a relatively low C content prevents the formation of a decarburized layer and ensures a strength of at least 190 kgf/mm.sup.2. The patent publication also discloses that the effects are further increased by adding elements such as V, Ni, Mo, Nb and B.
For the basic chemical composition system having a high C content, attempts having been made to prevent the decarburization and improve the properties of a spring steel by allowing the steel to contain from 0.05 to 0.1% of Se as disclosed in Kokai (Japanese Unexamined Patent Publication) No. 7-278747. As explained above, decarburization is commonly prevented to improve the properties of a spring.
For a spring prepared by cold working without considering decarburization, Kokai (Japanese Unexamined Patent Publication) No. 4-285142 discloses a process wherein many costly alloying elements such as Cr and Mo are added to the steel in combination in large amounts, the surface hardness of the steel is adjusted to up to Hv 400 by heat treatment, and the steel is subsequently nitrided and shot-peened to have a surface hardness of at least Hv 900 while the breakage of the steel is prevented during spring formation. As described above, technologies for the preparation of springs have generally been developed to cope with highly strengthening spring steels using such costly alloying elements.